1. Field of the Invention
This invention relates to a shutter and more particularly to a focal plane shutter of the type arranged to be electromagnetically operated.
2. Description of the Prior Art
Shutter devices of varied kinds employing an electromagnetic force as a drive source have been proposed as a result of recent developments in permanent magnets that give higher magnetic fluxes. However, for driving a focal plane shutter with such an electromagnetic device, it has been extremely difficult to arbitrarily set the travelling characteristics of shutter curtains.
Generally, a focal plane shutter is arranged to have its leading shutter curtain (or blades) and the trailing shutter curtain (or blades) driven by their own drive sources; and an exposure is effected for a period of time during which a slit formed between the leading and trailing shutter curtains passes across the front of a photo-sensitive film. Therefore, a difference between the travelling characteristic of the leading shutter curtain and that of the trailing shutter curtain would cause various parts of a picture to have different exposure times and would thus result in unevenness of the exposed portion of the photo-sensitive film.
The travelling characteristic of a shutter curtain is determined by the output characteristic of the drive source, the inertia and friction of the moving members of the shutter, etc. The relation of the travelling distance of the shutter curtain to the travelling time does not change linearly. Particularly, as is well known, the travelling speed is slow at the start. The speed of travel is then gradually accelerated as time elapses.
Before commencement of an exposure, the leading and trailing shutter curtains have their ends overlap each other to prevent light from leaking. They are thus arranged to have different start positions. The leading curtain has its start at a point away from the edge of an exposure window while the trailing curtain has its start at a point closer to the exposure window. Therefore, if the travelling characteristic of the leading shutter curtain is exactly the same as that of the trailing shutter curtain, the exposure time at an area of the picture around the exposure starting point would be short while the exposure time at another area of the picture around an exposure ending point would be long. This results in an uneven exposure.
To solve this problem, a conventional spring operated type focal plane shutter varies the output characteristic of a spring for driving the leading shutter curtain from that of a spring for driving the trailing shutter curtain. The trailing curtain has been thus arranged to accelerate more quickly than the leading curtain. In other words, the trailing shutter curtain has been arranged to have a faster rise time than the leading shutter curtain. Hence, the wire diameter, coil diameter and number of turns of one shutter curtain driving spring are arranged to differ from those of the other shutter curtain driving spring. In addition to that, at the time of assembly and adjustment work, the strength of each spring has been adjusted to prevent an unevenness of exposure from showing in a picture.
In cases where an electromagnetic device is employed as drive source and particularly where a focal plane shutter is provided with an electromagnetic drive source of the meter type having a moving coil or a moving magnet, it has been extremely difficult to make the adjustment mentioned above.